SER is mainly found in cells actively engaged in
(a) Secretion activity
(b) Proteins
(c) Lipid metabolism
(d) Catabolic activity

Options

A. Secretion activity
B. Proteins
C. Lipid metabolism
D. Catabolic activity

Answer

To determine the primary function of the smooth endoplasmic reticulum (SER), we can analyze the options provided:

1. **Understanding SER**: The smooth endoplasmic reticulum (SER) is a type of endoplasmic reticulum that lacks ribosomes on its surface, which is why it appears smooth. In contrast, the rough endoplasmic reticulum (RER) has ribosomes and is involved in protein synthesis.

2. **Functions of SER**: The SER is primarily involved in several key functions:
- **Lipid metabolism**: The SER plays a crucial role in the synthesis and metabolism of lipids, including phospholipids and cholesterol.
- **Detoxification**: It is also involved in detoxifying harmful metabolic byproducts and drugs.
- **Synthesis of steroid hormones**: The SER is important in the production of steroid hormones in certain cells.

3. **Analyzing the options**:
- **(a) Secretion activity**: While the SER can be involved in the secretion of lipids, it is not primarily known for secretion activities like the RER.
- **(b) Proteins**: This is primarily the function of the RER, which is associated with ribosomes and protein synthesis.
- **(c) Lipid metabolism**: This is the main function of the SER, as it is specifically designed for the synthesis and metabolism of lipids.
- **(d) Catabolic activity**: While some catabolic processes may occur, the SER is not primarily known for this function.

4. **Conclusion**: Based on the analysis, the correct answer is (c) Lipid metabolism, as the smooth endoplasmic reticulum is mainly found in cells that are actively engaged in lipid synthesis and metabolism.

### Final Answer:
(c) Lipid metabolism